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1119
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9/3/2019
Location
Georgetown, KY
US
Edited Date/Time
4/18/2023 11:27am
i bought a vhm head 12° piston combo, on paper it says squish should be 0.80
but doesnt descript if it should be in thousandths or in inches,
then on the dome box from vhm it says 0.80 mm
i set the squish to that this passed weekend my son went half a lap and it seized the big end rod bearing in half a lap.
0.80mm is 30 thousands
0.80 thousandths is around 2mm
what squish is the truth here?
0.030" is pretty tight
Running 110 Octane fuel 0.038" to 0.042" squish is my go to for MX, if running less than 110 octane you need to open squish up
the oem base gasket thicknessses equal to:
0.99mm
the base gasket thickness to give me 0.80mm squish with vhm head was:
0.82mm
so now im confused on what the hell is going on,
cause a 10th of a mm hell even two tenths of a mm is barely anything!
I am sure they meant 0.80mm. .080 inch (80 thousandths) would be 2mm which is ludicrously big squish. Barely have compression.
Also considering VHM is a European company and only the US uses inches for the most part.
0.80mm (30 thousandths of an inch) is not a crazy tight squish for a race 85. It sounds like something else caused the rod bearing to go. How many hours were already on the bottom end? What fuel, oil, and ratio are you running?
40:1
Rod Was Brand new 0 Hrs wossner billet rod.
Piston was new. Piston ring was new. Wrist pin was new. Gaskets were new. Crank seals New. Crank bearings New.
Everything was NEW... kinda sucks... haha
The Shop
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
Free shipping: VITALMX
Luxon 4-Post Bar Mounts
$189.95 - $239.95
So, there was an issue with the rod bearings going out on Wossner rods, I want to say about a year ago. Local builder here had a handful of them let go and had heard of others, all in Austrian 85's. It was basically becoming a known issue. I can't recall if it was only on their standard rod or also included the billet one. I put a billet one in a KTM 85 about 8 months ago and it had zero issues. Perhaps you got old stock of one of the bad bearings?
In any case, a squish of 0.80mm, in and of itself, should not cause a seized rod bearing. There's something else going on.
Minimum squish clearance is calculated off the stroke of an engine. 0.8mm would be fine for an 85. If it seized the bearing it wasn't a squish clearance issue.
We ran .028-.030" on our KTM 85's with no issues on 50/50 AvGAS/non-ethanol premium............
Currently have 160hrs on a '22 85sx, have always run 50:1 Lucas semi-syn with 91 Ca. pump gas at .032" squish. The original crank was rebuilt for preventative maintenance at 110hrs and was still tight.
Thank you for all the responses! Here are some pictures...
I would double check the gas from the tank on the bike with cardboard just to verify there was oil in the gas. I know.. I know "I am dead sure there is oil!!!" I would still check it. Takes 2 seconds.
yep, oil and lead tank was drained due to Tank wrap being applied: DEI insulation tape....
Oil was mixed 40:1 with fuel that was cut 25%leaded 110/ 75%shell 93.... with Bel ray H1R 40:1 mix..... then that tank was inserted into the bike... only mixed a gallon due to tank size....
It looks like the rod was spinning on the bearing, as if it was pressed to tight or the bearing or rod width were wrong. Is there any side clearance at all? Hard to tell from photos.
Mr Robinson is spot on. I usually shoot for .020" feeler gauge side clearance minimum
yes 27thousandths clearence checked with feeler guage
Interesting. Obviously, with the flattened out pins the bearing was not spinning on the pin for some reason.
By chance does anyone know the size of the original squish rm 85?
Hello.
Did you find the root cause for the seized rodbearing. I experienced exactly the same this weekend for my sons 2023 KTM. Wössner connecting rod seized after few minutes . Crankshaft rebuild in accordance to factory spec, 0.60mm axial play, and trued within spec (0.01mm)
This is the P2065-R rod. Wössner have not yet commented the issue.
BR
FritzP
Apologies for resurrecting an old topic but I'd really like to know how this was resolved.
I'm suffering the same thing. I bought a used 2022 KTM 85 that had just recently been rebuilt (Top and bottom)--receipts to prove it.
Then over the course of the last 30 hours that I've owned it I've had to put two new cranks in because of they have seized. Each time it looks like a temperature related failure of the big end rod bearing that generates about 1/8" of end play.
I feel like I've tried everything and don't really want to go through the full rebuild process again to just experience the same thing.
Thanks!
Pit Row
Give us more info. What brand were the 2 rods that failed? What oil and ratio are you running? I'm thinking there's either an assembly issue, poor quality cranks/rods, air leak, lubrication issue, or you're ingesting dirt.
1st crank was a Hot Rods and 2nd crank was a Wossner.
Background:
I think I'm going to get a lot of crap for running the "snowmobile" 2t oil but I've got over 200 trouble free hours using the same oil on a 2023 KTM125xc. Also used to run this in my 300xc (before I joined the dark side and got a Stark Varg). I was introduced to this oil by a local pro racer and have really liked it in everything else. Burns super clean and doesn't smell.
But I really can't think of anything else that would possibly be causing my issue? Any ideas? Really appreciate the help.
Good info. My initial thoughts.... I tuned for a fast kid that went through about 6 of these bikes in 2021 and 2022. I was never below a 142 main or a 45 pilot at any time and we rode from sea level to 5,000ft and very high AZ temps and the bikes were jetted to crisp perfection. We put over 160 hours on each of the bikes and never had a single engine issue. We did crank rebuilds every 80 hrs using ProX or Wossner rods on the OEM cranks. We used Motorex Cross Power 2T at 40:1 exclusively.
Not saying anything you are doing is wrong. But this is what worked for us just for comparison. I'd start with an OEM crank and would pressure test the engine carefully for air leaks. I don't know anything about that oil you are using and I would not write it off simply for being a snowmobile oil, but if it was me I would go with an oil that is known to be zero issues in these bikes just to rule it out. What pistons are you using? I always stuck with the Vertex cast pistons and had zero issues. Perhaps a forged piston might be a tad heavier and cause rod issues?
Thanks for your feedback. I don't have the stock crank but my plan is to rebuild this Wossner with a Pro-X rod. I'll be sure to do a leak down test once I have it back together.
I'm going to transition to Maxima Castor 927 at 32:1.
My jetting is bang on for my altitude and temp (Utah, 5000-8000ft and 80 degrees):
Main: 138
Pilot: 42
Needle: #3
Airscrew: 1-1.5 turn
Be aware, all the jetting specs in the manual assume you are using the recommended 40:1 from the manual. If you go 32:1 it leans the bike out a hair more than 40:1.
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